Symbols are more than variables. Symbols and Property Lists. Symbols are more than variables. Symbols are more than variables.

Transcription

1 Symbols are more than variables So far we have been looking at symbols as if they were variables Symbols and Property Lists Based on Chapter 7 in Wilensky + Prof. Gotshalks notes on symbols > ( setq x 3 ) 3 > x 3 LP-1 LP-2 Symbols are more than variables Symbols are more than variables Symbols have a complex structure Symbols are implemented as a record structure with fields So far we have only used the value field Name Symbol Value Property list Package Definition Print name Each time Lisp creates a symbol, it creates a pointer location for each of the components LP-3 From Harrison pages LP-4

2 Symbol components - value The value of a symbol Access by ( symbol-value symbol ) Also by typing symbol in the interpreter > ( setq x 3 ) 3 > ( setq y ) Symbol components - name The name Lisp uses when printing s-expressions containing the symbol In gcl it is all uppercase Access by ( symbol-name symbol ) All symbols having the same name are the same symbol > ( + ( symbol-value x ) ( symbol-value y ) ) 8 LP- LP-6 Symbol components - name Symbol components - definition > ( setq var1 12 ) 12 > (setq VaR1 11) 11 > var1 11 > ( symbol-name var1 ) VAR1 > ( symbol-name VaR1 ) Contains a function definition, macro definition or a special form with symbol (name) as its name So far we have seen ( defun symbol ( Can use the function fboundp to see if a symbol has a definition Access by ( symbol-function symbol ) VAR1 LP-7 LP-8

3 Symbol components - definition Symbol components - package > ( setq x 4 ) 4 > ( defun x ( a b ) ( + a ( car b ) ) ) X > (symbol-value x) 4 > ( symbol-function x ) (LAMBDA-BLOCK X (A B) (+ A (CAR B) ) ) > ( fboundp x ) T A package is an interface between the symbol names and the internal representations of the symbols stored by Lisp If a symbol is defined in a package it is represented uniquely in that package The package component for a symbol points to the package it belongs to Access by ( symbol-package symbol ) > ( symbol-package + ) #< LISP package> LP-9 LP-10 Symbol components property list A list of attribute-value pairs Access by ( symbol-plist symbol ) ( set symbol form ) MORE TO COME!!! set is a function Symbol is evaluated, value of symbol is set to form Value is returned LP-11 LP-12

4 ( setq <symbol form> ) ( setf <place form> ) setq is a special function Symbol, which is not evaluated, is set to form The value of the last form is returned setf is a macro Stores the value form in the location referred to by place The value of the last form is returned When the first argument to setf is a symbol name, place refers to the value component (i.e. works the same as setq) but place can refer to locations other than the value cell of a symbol LP-13 LP-14 > (setq x (a b c) ) > (set (car x) ) >X > a > (setq x (a b c) ) > (setf (car x) ) >X ( B C) > a Error: The variable A is unbound > (setq x (a b c) ) > (setq y (cons (car x) (cdr x) ) ) >x > y LP-1 LP-16

5 > (setf (car x) ) > x ( B C) >y > (setf (cddr x) 20) 20 What is a Property List? Think of symbols as real-world objects e.g. a book A symbol can have a number of properties each with their own value Properties of a symbol are like properties of the object it represents e.g. title, author, number of pages Value of an attribute in a property list can itself by a property list > x > ( B. 20) > y > (A B. 20) LP-17 LP-18 An actual Lisp list What is a Property List? Consists of alternating properties and corresponding values Lisp models the book example above with» symbol book1» property list ( title IRobot author Asimov pages 412 ) Accessing properties Use ( get symbol propname ) to access a property value for a symbol» Assume the symbol purse has the property list > ( colour white change ( penny 3 dime 4 looney 6 toonie 10 ) ) In general, property lists are often though of as simple databases» Then > ( get purse change ) > returns the S-expression ( penny 3 dime 4 looney 6 toonie 10 ) LP-19 LP-20

6 Setting Properties Property List ambiguity Use ( setf ( get symbol propname ) value ) to set a property value» ( setf ( get purse colour ) pink ) > changes the colour of the purse to pink» the get returns the address of where the attributevalue is» Value is the new value of the property If a property does not exist get returns nil What if a property value is nil? LP-21 LP-22 Property list ambiguity 2 Good mathematical and programming practice is to give a special name for nil property values. Could use (change none) in place of (change nil) Then nil would mean the property does not exists as opposed to the value of an existing property is nil OR Can use a third optional argument in a call to get ( get book1 title unknown ) The third argument is the value to return should the search for the property fail adam is a function that returns the most distant male ancestor of a person through the paternal line working through a father property LP-23 LP-24

7 Create a database: (setf (get 'colin father) 'michael) (setf (get 'david father) 'colin) (setf (get 'ryan father) 'colin) (setf (get 'steve father) 'david) (setf (get 'nathan father) 'ryan) (setf (get 'frank father) 'harvey) (setf (get 'joey father) 'frank) (setf (get 'mark father) 'frank) (setf (get 'ross father) 'mark) michael colin david ryan steve nathan harvey frank joey mark ross LP-2 LP-26 > (adam steve) michael > (adam ross) harvey > (adam michael) (defun adam (x) (if (get x 'father) (adam (get x 'father) ) x ) ) michael LP-27 LP-28

8 Property Lists and Scope Unattached property lists > (setf x 4) 4 > (setf (get 'x 'colour) 'red) RED > (defun x ( x ) (setf (get 'x 'colour) 'black) (setf x 6) ) X > (x 'x) 6 > x Property lists do not need to be associated with a symbol s plist» Any list of attibute-value pairs will do > ( setf x '( colour blue change ( penny 4 dime ) ) > ( getf x 'change ) returns ( penny 4 dime )» Even just a property list structure will do > ( getf '( colour blue change ( penny 4 dime )) change )) returns ( penny 4 dime ) Note use of getf instead of get 4 > (get 'x 'colour) BLACK LP-29 LP-30 Association lists Like property lists Associate attributes with values Uses lists of lists» ( (colour black) (size large) ) First of each sublist is the property (key) and the second is the value. > (setf sarah ( (height 4) (weight 4.4) ) ) ( (HEIGHT 4) (WEIGHT 4.4) ) > ( assoc height sarah ) (HEIGHT 4) LP-31